Wave particle dualtiy and neutron diffraction

1. A neutron has a kinetic energy of 10MeV. What size object is necessary to observe neutron diffraction effects?

Is there anything in nature of this size that could serve as a target to demonstrate the wave number of 10MeV neutrons?

K= 10MeV or 1.602 x 10^-12J Mass of neutron = 1.675 x 10^-27kg

This probably should be a simple problem but for some reason i've not been able to figure out how to solve for the size of the required object. What part of that problem gives the info needed to solve for the size of an object required to observe diffraction?